Investigation of CL‐20 and RDX Nanocomposites

Nanocrystalline explosives offer a number of advantages in comparison to conventional energetics including reduced sensitivity and improved mechanical properties. In this study, formulations consisting of 90 % hexanitro‐hexaazaisowurtzitane (CL‐20) or cyclotrimethylene trinitramine (RDX) and 10 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 200 nm to 2 μm. The process to create these materials used a combination of aqueous mechanical crystal size reduction and spray drying. The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy, X‐ray diffraction, and Raman spectroscopy. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. In the most extreme case (involving CL‐20/PVOH formulations), crystal size reduction from 2 μm to 300 nm improved compressive strength and Young’s modulus by 126 % and 61 %, respectively. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nanoscale dimensions.     

Nanoenergetic Gas‐Generators: Design and Performance

Nanoenergetic gas‐generator (NGG) mixtures may have various potential military applications from aircraft fuels to rocket propellants, explosives, and primers. To find reactions generating the highest pressure peak, we studied eight nanoenergetic reactions. The Al/Bi₂O₃ reaction generated the highest pressure pulse among the eight thermite reactions. We developed a highly efficient, one step process for synthesis of Bi₂O₃ nanostructured particles. Its use generated about a three times higher peak pressure (∼10 MPa) than when using commercial bismuth oxide nanoparticles (3 MPa). The pressure in the vessel rose very rapidly to a peak value for a duration of ∼0.02 ms and ΔP/Δt of up to 500 GPa s⁻¹. Increasing the crystallinity of the bismuth oxide nanoparticles increased the peak gas pressure by 25%. The maximum pressure×volume (PV) value obtained at m=0.1 g with our synthesized Bi₂O₃ was 707 Pa m³ much higher than that reported in the literature (33 Pa m³) for the same sample mass. Addition of carbon to the reactant mixtures did not increase the peak pressure. Addition of up to 3 wt.‐% of boron to the thermite systems increased the peak pressure by ∼50%.     

Investigation of HMX‐Based Nanocomposites

Advanced munition systems require explosives which are more insensitive, powerful, and reactive. For this reason, nano‐crystalline explosives present an attractive alternative to conventional energetics. In this study, formulations consisting of 95 % octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine (HMX) and 5 % polyvinyl alcohol (PVOH) were prepared with mean crystal sizes ranging from 300 nm to 2 μm. The process to create these materials used a combination of mechanical particle size reduction and spray drying, which has the advantages of direct control of crystal size and morphology as well as the elimination of ripening of crystals (which occurs during slurry coating of nanomaterials). The basic physical characteristics of these formulations were determined using a variety of techniques, including scanning electron microscopy and X‐ray diffraction. Compressive stress‐strain tests on pressed pellets revealed that the mechanical properties of the compositions improved with decreasing crystal size, consistent with Hall‐Petch mechanics. The 300 nm HMX/PVOH composition demonstrated a 99 % and 129 % greater strength and stiffness, respectively, than the composition with 2 μm HMX. The formulations were subjected to the Small Scale Gap Test, revealing a significant reduction in shock sensitivity with decreasing crystal size. The formulation containing 300 nm HMX registered a shock initiation pressure 1.6 GPa above that of the formulation with 2 μm HMX, a 44 % improvement in sensitivity. These results serve to highlight the relevance of structure‐property relationships in explosive compositions, and particularly elucidate the substantial benefits of reducing the high explosive crystal size to nano‐scale dimensions.     

Reactive wave propagation in energetic porous silicon composites

A parametric study of reactive wave propagations in porous silicon (PS) – oxidizer composites is presented. This study investigates the effects of the composite equivalence ratio and the oxidizer, and also the nanoscale and microscale structure, and the effect of dopant atoms, which are specific to this nanostructured composite material. The reactive wave speed and structure for energetic PS composites formed by depositing sodium, magnesium, or calcium perchlorates within the nanoscale pores were analyzed with high speed video recordings and spectroscopic temperature measurements. The findings indicate that heavily doped samples that do not yield a microscale structure result in slow propagation speeds, and low doped substrates with randomly formed micro-crack patterns during the electrochemical dissolution result in high speed propagations. A systematic study of the mixture composition revealed very wide flammability limits and flame speed and temperature measurements independent of the equivalence ratio, consistent with thermochemical equilibrium calculations. Also, while all the composites considered in this study are fuel rich with equivalence ratios greater than 1.60, the composites with equivalence ratios closer to unity exhibited lower temperatures and propagation speeds than more fuel rich composites. This unusual behavior of the composites is attributed to the inhomogeneity of the system even though the reactants are mixed at the nanometer scale. This was illustrated by developing a phenomenological model describing the interaction of silicon and the oxidizer within a single nanometer scale pore, which revealed that the reactive wave propagation is more strongly controlled by the specific surface area than the global equivalence ratio, due to the diffusion length scales involved.     

Enhanced reactivity of nano-B/Al/CuO MIC's

Aluminum is traditionally used as the primary fuel in nanocomposite energetic systems due to its abundance and high energy release. However, thermodynamically boron releases more energy on both a mass and volumetric basis. Kinetic limitations can explain why boron rarely achieves its full potential in practical combustion applications, and thus has not replaced aluminum as the primary fuel in energetic systems. In particular, the existence of the naturally formed boron oxide (B₂O₃) shell is believed to play a major role in retarding the reactivity by acting as a liquid barrier if it cannot be efficiently removed. In this paper we demonstrate from constant-volume combustion experiments that nanoboron can be used to enhance the reactivity of nanoaluminum-based Metastable Intermolecular Composites (MICs) when the boron is <50 mol% of the fuel. It was also observed that an enhancement was not achieved when micronboron (700 nm) was used. Thermodynamic calculations showed that the aluminum reaction with CuO was sufficient to raise the temperature above ∼2350 K in those mixtures which showed an enhancement. This is above both the boiling point of B₂O₃ (2338 K) and the melting point of boron (2350 K). A heat transfer model investigates the heating time of boron for temperatures >2350 K (the region where the enhancement is achieved), and includes three heating times; sensible heating, evaporation of the B₂O₃ oxide shell, and the melting of pure boron. The model predicts the removal of the B₂O₃ oxide shell is fast for both the nano- and micronboron, and thus its removal alone cannot explain why nanoboron leads to enhancement while micronboron does not. The major difference in heating times between the nano- and micronboron is the melting time of the boron, with the micronboron taking a significantly longer time to melt than nanoboron. Since the oxide shell removal time is fast for both the nano- and micronboron, and since the enhancement is only achieved when the primary reaction (Al/CuO) can raise the temperature above 2350 K, we conclude that the melting of boron is also necessary for fast reaction in such formulations. Nanoboron can very quickly be heated relative to micronboron, and on a timescale consistent with the timescale of the Al/CuO reaction, thus allowing it to participate more efficiently in the combustion. The results indicate that sufficiently small boron can enhance the reactivity of a nanoaluminum-based MIC when added as the minor component (<50% by mole) of the fuel.     

Nano Aluminum Energetics: The Effect of Synthesis Method on Morphology and Combustion Performance

Nanoscale aluminum based energetic composites were prepared using polytetrafluoroethylene (PTFE) as an oxidizer, and optimized according to the maximum experimentally observed flame propagation rate in an instrumented burn tube. Optimization of the aluminum‐based composites was performed using nanometric aluminum from two manufacturers, Argonide Corporation and Novacentrix, and the combustion results represent the first direct comparison of these two materials in a burn tube configuration. Argonide aluminum was found to consist of many fused spheres of nano aluminum mixed with some larger micron sized particles. Novacentrix aluminum consisted of spherical particles with a closer particle size distribution. The propagation rate optimized wt.‐% aluminum powder values were 50 and 44.5 for Novacentrix and Argonide, respectively. At the optimized conditions, the time to steady propagation for both Argonide and Novacentrix were similar, however the startup time for the Novacentrix based mixtures was more sensitive to changes in the mixture ratio. The presence of micron sized aluminum and lower surface area, but higher active content in the Argonide mixtures resulted in lower propagation rates, pressurization rates and peak pressures but higher total impulse values. It was found that peak pressure is not the sole determining factor in propagation rate, but the highest pressurization rates correlate with propagation rate.     

Kinetic study of thermal- and impact-initiated reactions in Al-Fe2O3 nanothermite

A study on the kinetics of thermal- and impact-initiated chemical reactions in Al-Fe₂O₃ nanothermites prepared using self-assembly and solvent-based mixing techniques was conducted in this work. Thermochemical initiation using ignition wire, dynamic pressure tests, and differential thermal analysis showed significant enhancement in reaction kinetics for the thermite prepared by self-assembly, in contrast to those prepared by simple physical mixing of nano- and micro-sized powder precursors. The intimate mixing using self-assembly increases the interfacial contact area between Al and Fe₂O₃, which influences the thermochemical reaction initiation characteristics more so than the particle size effects. On the other hand, results of impact-initiation of reactions at velocities up to 400 m s⁻¹ reveal that the reactant particle size plays a more dominant role in the case of such mechanochemical processes. It was found that micron-size thermite powder mixture system reacts at a lower impact energy threshold than the mixtures of nano-sized powders prepared either by solvent-mixing or self-assembly. The difference in the reaction threshold is associated with the higher localized strain resulting from fewer interparticle contacts in micro-sized powders in comparison to more distributed strain achieved during impact-initiation of nano-sized thermite particle.     

Modeling of a reacting nanofilm on a composite substrate

This article provides a detailed computational analysis of the reaction of dense nanofilms and the heat transfer characteristics on a composite substrate. Although traditional energetic compounds based on organic materials have similar energy per unit weight, non-organic material in nanofilm configuration offers much higher energy density and higher flame speed. The reaction of a multilayer thin film of aluminum and copper oxide has been studied by varying the substrate material and thicknesses. The numerical analysis of the thermal transport of the reacting film deposited on the substrate combined a hybrid approach in which a traditional two-dimensional black box theory was used in conjunction with the sandwich model to estimate the appropriate heat flux on the substrate accounting for the heat loss to the surroundings. A procedure to estimate this heat flux using stoichiometric calculations is provided. This work highlights two important findings. One is that there is very little difference in the temperature profiles between a single substrate of silica and a composite substrate of silicon-silica. Secondly, with increase in substrate thickness, the quenching effect is progressively diminished at a given speed. These findings show that the composite substrate is effective and that the average speed and quenching of flames depend on the thickness of the silica substrate, and can be controlled by a careful choice of the substrate configuration.     

Formation of Al/CuO bilayer films: Basic mechanisms through density functional theory calculations

We carry out Density Functional Theory calculations of the initial steps of CuO deposition onto Al(111) surface and Al deposition onto CuO(11-1) surface to investigate the basic mechanisms responsible for the growth of Al/CuO interface. Chemical pathways for adsorption and incorporation into the subsurfaces are examined, and associated activation energies are determined. We demonstrate that Al does penetrate the CuO surface with subsequent amorphization of the CuO upper layer. In turn, CuO undergoes a dissociative adsorption onto Al(111), leading to isolated Cu and O atoms of which further penetration in the Al surface is detailed. While Cu pathway for subsurface penetration is characterized by a low activation barrier (0.5 eV), O interaction with the Al surface is much more complex; aluminum oxidation is shown to occur at a nominal oxygen coverage through a drastic rearrangement of the Al surface atoms.     

Combustion characteristics of novel hybrid nanoenergetic formulations

This paper presents the combustion characteristics of various copper oxide (CuO) nanorods/aluminum (Al) nanothermite compositions and hybrid nanoenergetic mixtures formed by combining nanothermites with either ammonium nitrate (NH₄NO₃) or secondary explosives such as RDX and CL-20 in different weight proportions. The different types of nanorods prepared in this study are referred to as CuO-VD (dried under vacuum at 25 °C for 24 h), CuO-100 (at 100 °C for 16 h) and CuO-400 (short time (1 min) calcination at 400 °C). The physical and chemical characteristics of these different kinds of CuO nanorods were determined using a variety of analytical tools such as X-ray diffractometer, transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR), surface area analyzer and simultaneous differential scanning calorimeter (DSC)/thermogravimetric analyzer (TGA). These measured characteristics were correlated with the combustion behavior of the nanoenergetic compositions synthesized in this work. The use of different drying and calcination parameters produced the synthesis of CuO nanorods with varying amount of hydroxyl (OH) and CH_n (n = 2, 3) functional groups. The experimental observations confirm that the presence of these functional groups on the surface of CuO nanorods enabled the formation of assembled nanoenergetic composite, upon mixed with Al nanoparticles. A facile one-step synthesis of assembled composite through surface functionalization is reported and it can be extended to large-scale preparation of assembled nanoenergetic mixtures. The combustion behavior was studied by measuring both combustion wave speed and pressure–time characteristics. Pressurization rate was determined by monitoring the pressure–time characteristics during the combustion reaction initiated by a hot wire in a fully-confined geometry. Different amounts of nanothermite powder were packed in the same volume of combustion chamber by applying different packing pressures and the pressure–time characteristics were measured as a function of varying percent theoretical maximum density (% TMD). The experimental setup used in this work enabled us to study the functional behavior of initiating explosives such as NH₄NO₃ nanoparticles, RDX and CL-20 using nanothermites under fully-confined test geometry. The dent tests performed on lead witness plates support the experimental observations obtained from pressure–time and combustion wave speed measurements of hybrid mixtures.